Influence of Preparation Method on the Metal Cluster Size of Platinum/ZSM-5 Catalysts as studied with EXAFS.

Abstract

The size of Pt particles introduced into the channels of ZSM-5 zeolite by two different preparation methods has been studied with EXAFS. ZSM-5 was loaded with 2 wt 5% Pt by ion exchange and by impregnation. By careful calcination of the catalysts the dispersion was maintained. The EXAFS measurements were performed on the reduced system. Analysis of the data showed that in the impregnated sample the platinum was present in particles which on average were smaller than the platinum particles in the ion-exchanged sample. These results corroborate earlier high-resolution electron microscopy measurements performed ex situ on the calcined catalysts. Moreover, the average particle size found from EXAFS analysis agrees well with the HREM results, indicating that dispersion is maintained during reduction. Both techniques indicate particles that exceed the dimensions of a zeolite cage at the intersection of the channels. The fact that the impregnated system has smaller platinum particles than the ion-exchanged catalyst can be explained by the presence of additional potassium species in the pores of the impregnated zeolite. These species inevitably will hinder the migration of platinum to form larger particles.